On the model shown your best bet is to disconnect the light from inside the case. Also depending on the model, you may lose the IR receiver and battery level indicator as well.

Or if you don't want to disconnect the cable, you could have a physical barrier - some aluminum tape (actual aluminum, not grey/silver plastic) when cut to shape should stick there for some time and blend in somewhat better than other tapes.

Keep the lid open so the ambient light sensors get a reading. They will adjust the brightness accordingly in a dark room but not completely turn it off.

Changing power management setting from sleep to hibernation (suspend to disk) should turn it off completely at the cost of having to wait for writing the sleepimage file when closing and reading it into memory when waking up.